Bimodal Slip Segmentation of the SW Hellenic Megathrust Revealed by the Mw 6.8 Methoni Earthquake Sequence and Tomography

The influence of geological and structural heterogeneity on the megathrust slip behavior in subduction zones has been widely discussed for both the lower- and upper-plates. In this work, we present OBS local earthquake tomography on the outer forearc domain of the SW Hellenic subduction zone performed with the aftershocks of the 2008 M_w 6.8 Methoni subduction event. The latter initiated at a trench-normal sub-vertical slab tearing fault and we explore here the potential role of upper-plate structural heterogeneity. Our results reveal the contrasted nature of juxtaposed forearc upper-crustal blocks on top of the megathrust down to 30 km depth, inherited from the evolution of the Hellenic accretionary margin, and possibly laterally separated by nappe's contacts. The rupture extent of the mainshock and its major M_w 6.5 aftershock correlate with the low- and high-P-velocity crustal blocks and the sharp contact in between, and is downward limited by the 27–30 km deep mid/lower crust boundary. We propose a slip segmentation of the SW Hellenic megathrust that may result in moderate-size earthquakes controlled by (a) the small-scale upper-plate segmentation by crustal tectonic discontinuities and (b) the larger-scale lateral segmentation of the lower plate by trench-normal active slab tearing which reduces the occurrence probability of M > 7 earthquakes. However, our results do not preclude the occurrence of mega events; rather, they suggest that these may be less frequent or require specific conditions to rupture through segment boundaries. The correlation between crustal aftershock activity and Vp heterogeneity highlights the active internal imbrication of the upper plate. Inherited crustal blocks' scale architecture constitutes a possibly crucial factor that impacts megathrust seismic behavior, making it a key element for improving our understanding of seismic hazard at subduction zones.